The somatic sensory systems of the spinal cord, brainstem and cerebral cortex are being investigated in mammals. The objective of the research is to determine the functional organization of the cerebral cortical and brainstem descending pathways to the spinal cord which are involved in the integration of sensory information. In particular, the influences of the corticospinal and brainstem descending tracts on the activity of spinothalamic, spinocervical, spinocerebellar and other ascending sensory tract systems are being investigated with electrophysiological techniques. Anatomical studies employing the axoplasmic transport of tracer proteins are being used to determine the origin and terminations of the long descending and ascending tracts to the spinal cord.